The present invention relates to an improvement in a bipolar semiconductor device and a method of manufacturing the same.
Various conventional techniques have been proposed to improve the high-speed, high-frequency characteristics of a bipolar semiconductor device. Examples of these techniques include selective epitaxy and a combination of a super self-aligned structure, wherein polycrystalline silicon is used as an electrode for connection with a base region, and trench isolation technique. The latter combination is described, e.g., in the following: "Gigabit Logic Bipolar Technology, Advanced Super Self-Aligned Process Technology", Electronics Letters, Vol. 19, No. 8 (1983); "1.25 .mu.m Deep-Groove-Isolated Self-Aligned ECL Circuits", ISSCC82, IBM; and S. K. Wiedmann, "Status and Trends of I.sup.2 L/MTL Technology", IEDM Tech. Dig., pp. 47-50 (1983). Selective epitaxy is described, e.g., in H. J. Voss et al., "Device Isolation Technology by Selective Low-Pressure Silicon Epitaxy", IEDM Tech. Dig., pp. 35-38 (1983). N. Ohuchi et al., "A New Self-Aligned Transistor Structure for High-Speed and Low-Power Bipolar LSIs", IEDM Tech. Dig., pp. 55-58 (1983) describes a technology using both techniques mentioned above.
A bipolar transistor of a super self-aligned structure has been proposed which uses polycrystalline silicon as an electrode for connection with the base. Such a bipolar transistor is described, e.g., in N. Sasaki et al., "Bipolar Process Technology Evaluation by 3-Dimensional Device Simulation", IEDM Tech. Dig., pp. 546-549 (1983).
In the high-speed bipolar semiconductor devices described in the above texts, base resistance rbb influences the operation speed of the circuit most (i.e., it has the largest sensitivity coefficient) among device constants (see Sotomura and Yamazaki, "Development and Mass Production of Ultra High-Speed ECL Logic HD100K Series", Hitachi Heyoron, pp. 59-62 (1982)). Therefore, it is desirable to further reduce the base resistance rbb in order to improve the operation speed and high-frequency characteristics of the bipolar transistor.
When polycrystalline silicon is used as an electrode for connection with a base region, however, further micropatterning of presently available semiconductor devices is difficult since the sheet resistance of polycrystalline silicon is several tens to several hundreds of .OMEGA./.quadrature..
According to another conventional technique, a high-melting metal or a silicide thereof such as MoSi.sub.2, which has a low sheet resistance, is used in the super self-aligned structure as an electrode for connection with the base in place of polycrystalline silicon, thereby decreasing base resistance rbb. However, these techniques are not practical because ohmic contact between such asn alternative material and the base region is difficult to obtain. In addition, it is difficult to use these alternative materials as diffusion sources for the formation of external base regions.